Polydimethylsiloxane fluids may be conveniently prepared by any of several synthetic methods. A common and widely used industrial process to prepare such fluids involves the hydrolysis of halo-functional silanes followed by condensation. A second process begins with cyclic organosiloxanes utilizing a ring-opening polymerization to produce either a kinetically or equilibrium controlled mixture of linear and cyclic organosiloxane compounds.
Ring-opening polymerization of cyclic organosiloxanes may be accomplished by either acid or base catalysis. The catalysts that have been successfully employed range from very acidic catalysts such as trifluoromethane sulfonic acid to very basic such as potassium hydroxide or potassium silanolate. A wide variety of acid catalysts have been employed to catalyze ring opening polymerization, sulfuric acid, acid washed clays, acidic ion exchange resins and linear phosphonitrilic chlorides (LPNC).
While ring opening polymerization may be accomplished with either an acidic or basic catalyst, the preparative chemistry of hydrogen containing siloxanes (i.e. silyl hydrides) is restricted to the acidic catalysts. When a basic catalyst is used, the ring opening polymerization proceeds, but base catalyzed hydride abstraction produces hydroxy functionalities in place of the hydrogen functionalities and the material condenses through the silanol groups. While this produces a polymer, it produces a cross-linked polymer in contrast to a linear polymer.
Process considerations in the choice of an acidic catalyst for the preparation of hydrogen organosiloxanes tend to require the milder acid catalysts in contrast to sulfuric acid and trifluoromethane sulfonic acid because these acids are very strong and highly corrosive. The use of such strong acids requires the use of special alloys in process vessels to avoid acid induced corrosion and contamination of the resulting product.
Milder acid catalysts such as the acid washed clays and acidic ion exchange resins possess drawbacks that while avoiding the corrosion and contamination problems associated with strong acid attack on metal process vessels, cause other problems. The acidic ion exchange resins do not maintain catalytic activity well for any significant and economically useful period of time, requiring frequent regeneration or refreshment. Acid washed clays are generally used as powders to improve contacting efficiency between the reaction substrate and the catalyst which necessitates a downstream filtration to remove the acid washed clay catalyst fines from the product Further, acid washed clays generally contain residual amounts of water that contributes to a hydride silanol interchange that results in a gradual and undesired condensation polymerization of the hydride product. By comparison to the stronger acid catalysts, these milder acid catalysts suffer from lower reaction rates and thus a lower production of product per unit time at any given temperature.
While the kinetic rate deficiencies of any given catalyst may be offset by an increase in temperature, this solution has at least two serious drawbacks. The first is that as temperature is changed, i.e. increased, the relative proportions of reactants, desired products and undesired by-products change. This change may either benefit the desired process or be a detriment depending on the relative amounts of the desired product as a function of the increased temperature because the equilibrium constant for the reaction is a function of temperature. As the temperature is increased, the amount of energy furnished the reaction to increase the temperature must be increased (for endothermic reactions) and this almost always adversely affects the process economics. There is thus a complex balancing between the desired reaction rate, the desired product mix, catalyst activity and process operating variables.
In contrast to the acid catalysts that must either be neutralized, e.g. sulfuric acid, or separated from the product, e.g. acid washed clays, phosphonitrilic halides, particularly linear phosphonitrilic chlorides (LPNC), have found particular use for the redistribution and condensation of organosiloxane oligomers. These LPNC catalysts may be used at fairly low levels in the reaction being catalyzed, e.g. between 25 and 2,000 ppm. An additional advantage is that the catalyst may be left in the product and thermally deactivated if desired. This procedure usually does not result in any significant contamination of the product.
While the LPNC catalysts have been particularly useful for redistribution and condensation reactions involving silicones and siloxanes, they have not usually been used for ring opening polymerization because of the low rates associated with these catalysts in reactions of this type. While it is possible to achieve acceptable reaction rates in the synthesis of hydride siloxane organosiloxane copolymers when the hydride level is above approximately 1,000 ppm, the rate of ring opening polymerization in the presence of a low hydride level siloxane (.about.300 ppm) is extremely slow requiring a matter of days as opposed to hours. Thus LPNC materials would not be expected to be particularly well suited to catalyze ring opening polymerization in the presence of low hydride content siloxanes to make low hydride content siloxane polymers.